The present invention relates to a medical appliance for ultrasonic therapeutic treatment and/or other operation upon living body tissue.
The application of ultrasound in diagnostic scanning techniques and therapeutic treatment of specific medical conditions has been widely reported in the technical literature over the last 20 to 30 years. However, we have found no relevant reference to the use of frequencies in the range 30 to 100 kHz. The reported applications of therapeutic ultrasound relate almost exclusively to use of frequencies in the MHz band. This stems from the concept that therapeutic treatment using ultrasonic energy should be directed accurately to a well defined region of tissue and that this is best achieved with a finely focused beam, which in turn dictates the use of high frequencies. Similar arguments apply to the selection of frequencies for ultrasonic imaging applications. For example, the characteristic wavelength corresponding to a 3 MHz transmission through soft tissue is about 0.5 mm; but at 40 kHz, the wavelength would be approximately 37.5 mm.
Furthermore, it is known that the attenuation of ultrasonic waves increases with increasing frequency. The general effect of high frequency transmission is to produce relatively high energy absorption rates close to the entry surface, and for the effect to fall off with increasing depth. It might therefore be concluded that, for a given power input, there is a greater likelihood of potentially harmful side effects in tissue near the surface than for a corresponding low-frequency input when treating deep tissue injuries. This consideration becomes very important since in order to transmit enough energy to the required region, the risk of excessive absorption in surface layers may become unreasonably high when applying therapeutic ultrasound in the MHz band. For this reason, energy levels are limited by the requirement that power input should not exceed 3 watts/cm.
By selecting an operating frequency band in the range 30-130 kHz, a good wave penetration through deep muscle tissue is ensured and frequencies which are known to result in high attenuation in bone tissue are avoided.
Experience with this form of energy reveals a need to effect treatment in regions of the body which naturally inhibit access, particularly the hands and feet. In such cases, the use of a specially shaped therapy head may greatly facilitate the treatment of an injured joint. It is recognized that under such circumstances repetitive cyclic movement of the treatment head over the skin surface may be difficult and that the need to avoid the establishment of standing waves must therefore be satisfied by an alternative technique.